Optical pickup apparatus

Abstract

An optical pickup apparatus which, assuming that a distance from a first light source to a first grating is a, a distance from a second light source to a second grating is b, a first predetermined grating pitch is I, a second predetermined grating pitch is J, the wavelength of a first laser beam is λ 1 , and the wavelength of a second laser beam is λ 2 , satisfies λ 1/λ2 >(I·b)/(J·a), and which is capable of preventing a drop in the level of a tracking error signal resulting from decentering of an optical recording medium, even in the case of using either of optical recording media of different recording densities.

What is claimed is:  
     
       1. An optical pickup apparatus, comprising: 
       a first light source for emitting a first laser beam having a first wavelength;  
       a second light source for emitting a second laser beam having a second wavelength that is longer than the first wavelength of said first laser beam;  
       a first grating having a diffraction grating formed with a first predetermined grating pitch, for passing through said first laser beam as a main-beam, and for outputting two sub-beams composed from diffracted lights of said first laser beam;  
       a second grating having a diffraction grating formed with a second predetermined grating pitch different from said first predetermined grating pitch, for passing through said second laser beam as a main-beam, and for outputting two sub-beams composed from diffracted lights of said second laser beam; and  
       an optical system for guiding the main beam and the two sub-beams to a recording surface of a recording medium, and for guiding the main beam and the sub-beams reflected at the recording surface of said recording medium to a photodetector to form respective spots corresponding to the main-beam and the two sub-beams reflected, on light-receiving surfaces of said photodetector,  
       wherein assuming that a distance from said first light source to said first grating is given by a, a distance from said second light source to said second grating is given by b, said first predetermined grating pitch is given by I, said second predetermined grating pitch is given by J, the wavelength of said first laser beam is given by λ 1 , and the wavelength of said second laser beam is given by λ 2 ,  
       
         
           λ 1 /λ 2 >( I·b )/( J·a )  
         
       
       is satisfied. 
     
     
       2. An optical pickup apparatus according to  claim 1 , wherein said photodetector has three four-section light-receiving surfaces on which the respective spots of the main beam and the two sub-beams are formed, and generates output signals in correspondence with the light intensity of each sectioned surface. 
     
     
       3. An optical pickup apparatus according to  claim 1 , further comprising a tracking error signal generator for generating a tracking error signal with the differential push-pull method in accordance with output signals of said photodetector when said first laser beam is emitted from said first light source, and for generating a tracking error signal with the three-beam method in accordance with output signals of said photodetector when said second laser beam is emitted from said second light source. 
     
     
       4. An optical pickup apparatus according to  claim 1 , wherein said first grating and second grating are constituted by a single diffraction grating element, a diffraction grating of said first grating is formed at one surface of said diffraction grating element, and a diffraction grating of said second grating is formed at the opposite surface of the one surface of said diffraction grating element. 
     
     
       5. An optical pickup apparatus according to  claim 1 , wherein said first light source and said second light source are formed as a single element.